This PDF file contains the front matter associated with SPIE Proceedings Volume 11798, including the Title Page, Copyright information, and Table of Contents.

Little did I know that when I published our first laser microbeam paper on chromosome surgery in 1969, that 50 years later I would still be doing that—trying to explain the inexplicable result we saw back then. The path to an answer of why cells with partially photon-ablated chromosomes could reproduce and maintain the photon-induced deletion in the clonal population of cells, was a path that was solely possible by the parallel development of additional tools in the photonic toolbox. Development of digital-based imaging technologies, GFP-based molecular fluorescent probes, and an evolution of optical manipulation tools (photonic scissors and tweezers) have been combined to provide a tantalizing answer to the question of why cells can divide and form clones following removal of a segment of their DNA.

Many viruses employ motors to translocate DNA into capsids. Previous reports raise questions if motor function depends on DNA sequence and a “B-A scrunchworm” model predicts that “A-philic” sequences that transition more easily to A-form would alter function. We use optical tweezers to measure translocation of phage, plasmid, and synthetic A-philic, GC rich sequences by the T4 motor. We observed no differences in motor velocities, even with A-philic sequences predicted to show higher rate, no changes in motor pausing, and only modest changes in slipping. To test more generally for sequence dependence, we conducted correlation analyses across pairs of events. No significant correlations in packaging rate, pausing, or slipping versus position were detected in repeated measurements with different DNA sequences. These studies suggest that the viral motor insensitive to DNA sequence and fluctuations in packaging velocity, pausing, and slipping are primarily stochastic temporal events.

Myosin is biological molecular motor that transduces the chemical energy ATP hydrolysis into force and/or motion to drives many forms of cellular motility from muscle contraction to cell division. While the transduction process has been extensively studied key details remain unclear. Most importantly the timing of the force-generating powerstroke relative to the release of phosphate (Pi), a product of ATP hydrolysis, is not clear and the source of intense debate. We examined the ability of single-headed myosin Va to generate a displacement (i.e. powerstroke) of an actin filament in a kinetic, fluorescent energy transfer (FRET) and single molecule laser trap assays while maintaining Pi in its catalytic site by introducing a mutation that impedes Pi-release from the catalytic site (S217A). Kinetic experiments confirmed that the Pi-release rate was slowed ~5-fold by the S217A substitution, and FRET experiments suggested that the mutation slowed myosin’s rate of attachment to actin. At the single molecule level Wild type myosin Va (WT) generated a 7 nm powerstroke at a rate of ~600/s and the S217A myosin generated a statistically similar size powerstroke (8nm) at a similar rate (600/s) despite impeding the release of Pi from the active site. These findings are consistent with myosin Va generating a powerstroke while Pi is still in the catalytic site, and therefore challenge the hypothesis that Pi-release gates, and therefore precedes, the powerstroke. Our findings imply that myosin’s key mechanical event is triggered by binding to the actin filament, and not by the biochemical event of Pi-release. This provides important new insight into the mechanism of energy transduction by myosin, and evolutionarily related motor proteins.

Integrated nanopore sensors enable label-free electrical detection of single molecular biomarkers using modulations in ionic current across a nanoscale opening in a solid-state membrane. Optical trapping can dramatically increase the rate of molecular detection and, thus, significantly speed up the sensing process. This is accomplished by optical delivery of microbeads that carry molecular targets that are specifically bound to their surface. Once trapped at the nanopore location, targets are released from the carrier bead and drawn through the pore by the large electric field that is present right at the pore location. We review the principle and implementation of this approach on an optofluidic chip. We report over 1000x improvement in sensing time and the application of these devices to detection of different molecular biomarkers and for infectious disease diagnosis of clinical samples.

Predatory bacteria are an exciting area of investigation with great promise. Their impressive ability to devour other bacteria make them ideal candidates for clearing pathogens from the environment, water supply, and even the body. However, scientists are still learning how these bacteria grow and consume their hosts. We are studying Bdellovibrio bacteriovorus, one of these predatory bacteria. We have been investigating the process of B. bacteriovorus attaching to prey cells of Escherichia coli in order to learn how they identify their prey, latch onto it, and then burrow into it. Measurements using atomic force microscopy (AFM) at different times during these processes have been made. Optical tweezers offer an alternative to the AFM for making measurements and learning about the attachment processes of B. bacteriovorus. Bacteria such as E. coli (1 – 2 µm long) and B. bacteriovorus (0.50 - 1 µm long) are a material and size that is easy to trap. Fluid solutions are the ideal medium for optical trapping measurements, and so it is readily possible to examine the bacteria during the attachment process. To this end, we trap one B. bacteriovorus cell in a single trap and move it close to a surface coated with E. coli, then move it away from the surface after increasingly long periods of time. This work shows preliminary measurements of attachment forces of predatory bacteria to their prey and correlates these measurements with those taken with the AFM.

There is a need for new methodologies to investigate cell apoptosis and recovery, cell adhesion, and cell-cell interactions in cellular biology and neurobiology. Such systems should be able to induce localized cell injuries and measure damage responses from single cells. In this regard, pulsed lasers can be used to produce Laser- Induced Shockwaves (LIS), which can cause cell detachments and induce cellular membrane injuries, by applying shear force in order of µN . Furthermore, since the resulting shear force can increase membrane permeability, chemicals and markers can then be transferred into cells non-invasively. Continuous-wave lasers can be used as Optical Tweezers (OT), to apply non-contact delicate forces, as low as 0.1f N , and deliver materials into cells, and also move the cells to different locations. In this paper, we introduce a combination of modalities to apply variable forces, from femto to micro newtons, to cells. Our system consists of a 1060nm continuous laser light source for OT and a 1030nm femtosecond pulsed laser for generating LIS. To have a direct measurement of changes in the cellular thickness and membrane dynamics, the cells are imaged under a Quantitative Phase Microscope (QPM). Our microscope is capable of Differential-Interference Microscopy (DIC) and Phase-Contrast microscopy (PhC) and fluorescent microscopy, making it a unique system for studying cell injuries.

The laser has biomodulation effects on blood properties thus could regulate the microcirculation. The laser-RBC interaction mechanism is unclear, whereas most studies provide statistical results and lack detailed observations of laser irradiation effects on blood rheology. This study is designated to probe the in vitro effects of 520 nm diode laser irradiation on red blood cells (RBCs) mutual interaction properties in spontaneous aggregation process in autologous plasma by optical tweezers (OTs), with an attempt to reveal the laser-RBC interaction outcomes at a single-cell level. The results preliminarily show that though the laser irradiation statistically inhibited the increase of the size of RBC aggregates compared with the non-irradiating group, the aggregation force between single RBCs increased slightly with the time of irradiation.

Studies of cell-extracellular matrix (ECM) interactions within fibrous systems such as collagen or fibrin are challenging, particularly if peri-cellular stiffness cannot be monitored. Here we present our light-based method for non-invasive patterning of molecular crosslinking combined with multi-axes optical tweezers active microrheology to map ECM stiffness landscapes. This method allows us to generate prescribed stiffness gradients and associated anisotropies, which model stiffness of the natural peri-cellular ECM. Patterned crosslinking induces strain hardening and measured stiffness gradients are in agreement with predicted strain fields. Migratory cells respond to these gradients as assessed by change in F-actin distribution and morphological properties.

This talk will introduce a novel, recently developed, instrument that combines light sheet microscopy, optical trapping and multiplane imaging in a single platform capable of imaging 3D cultures of live cell over long time courses and mapping to these images the local mechanical properties of the material surrounding the cells. The presentation will highlight the wide-range of samples this instrument can study and discuss the analysis involved to evaluate the rheological properties of the material when either the trapping potential or the samples are non-isotropic.

We present a bespoke new instrument for micro-mechanical sensing and imaging of 3D cell cultures. Our instrument combines optical tweezers and multiplane imaging for 3D optical microrheology, with light sheet microscopy for 3D fluorescence imaging. This integrated system allows us to visualise cells in their 3D environment while also mapping the physical properties of the extracellular matrix local to the cells. Such a holistic approach will allow for a wide range of fundamental questions in the life sciences to be addressed by effectively sensing the micro-world from a cell’s perspective.

We provide a multiple-sinusoid modulated optical tweezers based method to perform microrheology of a sample over a large frequency band in a one shot. So we feed the trapped particle with a square wave and a superposition of multiple-sinusoid of specific frequency and amplitude to achieve our purpose towards active microrheology. To maintain the SNR upto a certain level for the entire frequency range we increase the amplitude at high frequency in our setup. We measure the parameters of the complex fluid by extracting the phase responses at each frequency of the particle with a high SNR value compared to passive microrheology. We perform this method for a various concentrations of polyacrylamide-water solution and obtain a good agreement of the fluid parameters with the theoretical values.

Viscosity is an important property of out-of-equilibrium systems such as active biological materials and driven non-Newtonian fluids. Noninvasive viscosity measurements typically require integration times of seconds. Here we demonstrate measurement speeds reaching twenty microseconds, with uncertainty dominated by thermal molecular collisions for the first time. We achieve this using the instantaneous velocity of a trapped particle in an optical tweezer, combined with a structured-light detection system. This opens a pathway to new discoveries in out-of-equilibrium systems.

True random number generators are in high demand for secure cryptographic algorithms. Unlike algorithmically generated pseudo-random numbers, they are unclonable and non-deterministic. In this paper, we extract the white noise from stochastic Brownian Markov trajectories and use it to generate random numbers that qualify NIST standard tests of randomness. We trap colloidal particles in water using optical tweezers and record its confined Brownian motion in real-time. Next, in a two-step process, we use the initial section of incoming data to train and calibrate our iterative algorithm on the trap stiffness and viscosity of the solution based on the autocorrelation and power spectrum properties of the noise; then, we extract random arrays from the next section of the data. Interestingly, we get the best random number sequence for the best calibration. We test the random number sequence, which we have obtained, using standard randomness tests and observing the randomness to improve with increasing sampling frequencies.1 In the next steps, we extend this method to a wider class of processes, such as an optically trapped particle modulated by a square pulse or an external colored noise generated by an Ornstein Uhlenbeck process – we estimate the timescale of both the modulation and viscous effect using our algorithm.

FORMA (force reconstruction via maximum-likelihood-estimator analysis) addresses the need to measure the force fields acting on microscopic particles. Compared to alternative established methods, FORMA is faster, simpler, more accurate, and more precise. Furthermore, FORMA can also measure non-conservative and out-of-equilibrium force fields. Here, after a brief introduction to FORMA, I will present its use, advantages, and limitations. I will conclude with the most recent work where we exploit Bayesian inference to expand FORMA's scope of application.

Photonic Force Microscopy (PFM) uses optically trapped particles to measure fluctuation based forces and potential landscapes. This feature makes it a very soft probing technique that is well suited to investigate surface interactions with biological samples. In addition, PFMs can be used in scan mode to create surface-profiles with a resolution better than the optical diffraction limit. In this talk we present an improved intermittent contact (tapping) mode PFM and a revised data analysis for more detailed surface height profiles.

We are interested in the opto-mechanical manipulation of nanodiamonds with a high concentration of SiV centers. Optical tweezers usually rely on the interaction of light and the ‘bulk’ polarizability of the nanoparticle itself. Here we exploit the polarizability of electronic resonances of optical centers embedded in the solid-state matrix to enhance the optical forces. This effect becomes particularly relevant for ensembles of active centers. The emitters being closely packed in a sub-wavelength volume, can act cooperatively, enhancing further the optical forces. This opens the possibility to apply the powerful toolbox of atomic physics for the quantum manipulation of ‘massive’ mesoscopic objects.

We present a generally applicable holographic force measurement method to simultaneously measure all components of the individual forces applied to a specific particle in a trapped ensemble, or to a specific site of an extended object. We show individual force measurements for up to ten traps. As this method does not require information about size, shape, or optical properties of the particles, it is well suited to study biological specimen. We demonstrate disentangling the individual forces on a red blood cell stretched by four optical traps. The detailed information (amplitude, phase, and polarization) about the scattered light field, which we gain by our method, allows us to also uncover the optical torque exerted on trapped particles. We experimentally confirm that our method delivers accurate results in various settings.

Stochastic resetting is prevalent in natural and man-made systems, giving rise to a long series of nonequilibrium phenomena. Diffusion with stochastic resetting serves as a paradigmatic model to study these phenomena but lacked a well-controlled platform by which it can be studied experimentally. Here, we report the experimental realization of colloid diffusion and resetting via holographic optical tweezers. We provide the first experimental corroboration of theoretical results and measure the energetic cost of resetting in steady-state and first-passage scenarios. In both cases, we show that this cost cannot be made arbitrarily small because of fundamental constraints on realistic resetting protocols.

We have built an information engine that can transport a bead in a desired direction by using favorable fluctuations from the thermal bath. However, in its original formulation, the information engine generates a fluctuating velocity and cannot control the position of the bead. Here, we introduce a feedback algorithm that can control the bead’s position, to follow a desired trajectory. The bead can track the path if the maximum desired velocity is below the engine’s maximum average velocity. Measuring the range of frequency that the feedback algorithm can track, we find a bandwidth that is slightly lower than the corner frequency of the bead in the trap.

Rocking Brownian motors achieve directional motion in highly diffusive environments by combining an inherently asymmetric static potential energy landscape and an oscillating “rocking” force [1, 2]. Our nanofluidic imple- mentation [3] features a confining wall with a ratchet-type nano-topography and AC electric fields as driving force. The potential energy landscape is due to electrostatic interactions between the charged particles and wall surfaces in close proximity. Based on this scheme, we have developed a nanoparticle size-separation device [4]. Along the particle transport direction, one of the confining walls features a sawtooth-like topography with a superimposed slope. In turn, we obtain a ratchet-shaped potential with increasing potential energy barrier heights caused by the decreasing nanofluidic gap. A sharp drop in particle current occurs as the potential barriers increase. The drop happens the later, the smaller the particle size. This allows for a sequential separation of the particle suspension into multiple sub-populations. The device was modeled by solving the Fokker-Planck equation. We show that the physics of the separation mechanism is governed by the energy landscape under forward tilt of the ratchet. The separation resolution is thus only dependent on the applied force. Experimentally, we demonstrate the separation of spherical gold particles of nominal 80 and 100 nm diameters with an applied voltage of 3.5 V and a sorting time of 20 s. We achieve a resolution of 2 nm, in accordance with our simulation results.

In the optical levitation in vacuum, we proposed a framework to construct arbitrary non-thermodynamic equilibrium steady state based on the energy feedback control. In the experiment, we have realized different non-thermodynamic equilibrium steady states, including a high-quality phonon laser. This work will stimulate the study of different non-thermodynamic equilibriums with optical levitation in vacuum.

Optical Matter systems, particularly those created with nanoparticles, allow studying new optical physics phenomena, non-reciprocal forces and even many-body effects. This talk will introduce self-assembling nanoparticle-based optical matter machines and the inter particle electrodynamic interactions that give rise to structures that can do work. It will also introduce a new many-body mechanism for spin-to-orbital angular momentum conversion as studied by experimental measurements, electrodynamics-Langevin dynamics simulations and theory.

We investigate the nonlinear behavior of a colloidal particle suspension driven by light in an optical single feedback system. We show numerically and experimentally that the process of transverse pattern formation is fundamentally similar to self-organized dynamics in volumetric media as for instance photorefractive or liquid crystals, but obtains, however, an additional degree of complexity due to the stochastic nature of colloidal suspensions. Additionally, we demonstrate experimentally the ability to control the formation of emerging patterns by sculpting the Fourier spectrum of the optical system. Thus, we extend a deeply studied feedback configuration by an ensemble of soft-matter particles and will present insights into this fascinating field of self-driven light-matter interaction.

Optical control of mechanical motion of solid-state objects weakly interacting with the environment, referred to as optomechanics, continues to enable new, ground-breaking methods and applications in the area of ultra- weak force sensing and quantum technologies. The platform based on optically levitated nanoparticles in vacuum (referred to as levitated optomechanics) constitutes an entirely new type of light-matter interface, which provides a broad and an easy tunability of all the system parameters. However, the majority of the previously reported experimental achievements in this area have only dealt with a single levitated object. Here, we demonstrate for the first time scalability of the levitated optomechanics to systems containing up to tens of nanoparticles and provide a unique methodology for characterizing the system parameters and non-linear inter-particle interactions. This work represents the first and crucial step in accessing many-body dynamical effects in the classical and quantum regimes. In particular, it opens the door to the experimental studies of many-body stochastic thermodynamics and to the preparation of mesoscopic entangled states between relatively massive objects.

The integration of cold neutral atoms with nanophotonic circuits offers significant potential as a light-matter interface for a wide range of applications ranging from fundamental studies in quantum optics, quantum many- body physics, quantum networks. Here, we demonstrate atom-photon interactions using an efficiently-coupled nanophotonic microring circuit on a chip. Cold atoms are prepared above the chip, and directed to the near field region of a whispering-gallery mode of a microring resonator by an optical dipole potential. Delivered atoms interact with the mode of a resonant light field coupled to the resonator. This demonstration of efficiently guided atoms and their interaction with a microring photonic circuit paves the way towards realizing an on-chip chiral atom-light interface.

Highly symmetrical objects are important to a manifold of applications such as precise measurements and sensing. However, creating a perfect sphere proves to be experimentally extremely difficult, as gravity and the support deform the sphere from its perfect shape. In this work, we optically levitate a micro oil droplet and activate it as whispering gallery mode optical resonator. Our contactless resonator displays a sphericity of less than one Ångström for a 10 µm radius droplet and is therefore one of the roundest objects there is. Additionally, we demonstrate an unprecedented resonance enhancement defined by “optical quality” over “mode volume” and lastly, benefitting from the high sphericity, we lift degeneracies of order higher than 200.

Recently, after a decade of experimental and theoretical efforts, coherent scattering has enabled the motional ground state of levitated nanoparticles at room temperature. While this represents an important milestone towards the creation of mesoscopic quantum objects, coherent quantum control of levitated nanoparticles still remains elusive. A valuable but less stringent condition is the so-called strong coupling regime (SCR), where the optomechanical coupling strength between the mechanical motion of a particle and an external optical cavity exceeds the particle’s mechanical damping and the cavity linewidth, as has been demonstrated in opto- and electromechanical systems. Here, we demonstrate the SCR at room temperature between a levitated silica particle and a high finesse optical cavity. Normal mode splitting is achieved by employing coherent scattering. Our table top experiment offers numerous ways to tune the optomechanical coupling strength at room temperature.

We report on cooling the center-of-mass motion of a nanoparticle via a coupling between its motion and the optical field of a high finesse cavity. At the core of the experiment is a hybrid trapping potential obtained by overlapping an electrodynamic potential and an optical standing wave. By tuning the experimental parameters, the interaction between the particle and the cavity field can be made linear or purely quadratic in displacement. While both can be exploited for cooling the latter gives rise to a more interesting dynamic. Indeed, the resulting interaction generates a Van der Pol nonlinear damping, which is analogous to conventional parametric feedback where the cavity provides passive feedback without measurement. We show experimentally that like feedback cooling the resulting energy distribution is strongly nonthermal and can be controlled by the nonlinear damping induced by the cavity. As quadratic coupling has a prominent role in proposed protocols to generate non-Gaussian quantum states, our work represents a first step towards producing such states in a levitated system.

We present a clean, vacuum compatible method for loading nanoparticles into optical traps, based on laser-induced acoustic desorption (LIAD). We investigate the effect of the particle loading conditions and the optical trap power on trapping efficiency for an optical standing wave trap in vacuum. Furthermore, cooling all translational and rotational degrees of freedom of a levitated particle with anisotropic susceptibility is demonstrated. A silicon nanorod is trapped in the standing wave field in ultra-high vacuum. Due to the anisotropy of the susceptibility tensor, the nanorod has an enhanced interaction with the light field as compared to a spherical particle of the same volume. These non-spherical nanoparticles are also sensitive to the polarization of the light. We aim to cool the librational motion of the particle by controlling the polarization of the trapping light field and explore whether manipulation via polarization will produce more efficient cooling than other methods.

We demonstrate parametric feedback cooling of a levitated birefringent microsphere held in a circularly polarised Gaussian trap. We show efficient cooling of the birefringent microsphere to the temperature less than a millikelvin, which is orders of magnitude lower than that of a silica counterpart. We reveal that the transverse spin momentum present in circularly polarised light plays a role in the trapped particle dynamics, which exhibits a range of nonequilibrium phenomena, namely, (i) biased stochastic motion, (ii) coupling of mechanical modes, (iii) nonconservative rotation-translation coupling. We further discuss the temperature of the trapped particle in a nonconservative vacuum optical trap.

Directed propulsion of lightsails to relativistic speeds by laser radiation pressure demands for passive stabilization, which can be achieved with subwavelength metasurface elements. Our tailored nanophotonic design based on silicon nitride is predicted to give rise to restoring forces and torques upon small perturbations in its dynamics. By measuring the angles and intensities of the diffracted orders, we infer the light-induced pressures on our lightsail prototype. Using Michelson interferometry and laser deflection spectroscopy, we can directly observe the dynamical stability of spring-supported suspended membrane designs. Our results pave the way for lab-scale experiments on optical manipulation of microscopic lightsails.

Experimental measurements related to the opto-mechanical stability of thin film diffractive beam-riders are discussed. Our theoretical predictions of radiation pressure forces indicate that these structures allow a perturbed laser-driven light sail to remain in the beam path. Radiation pressure forces of both a liquid crystal polymer bi-grating and an etched photoresist axicon diffraction grating will be described. Our experiments made use of a vacuum torsion oscillator having sub-nano-Newton sensitivity. The parametric damping of both systems will also be described. Our measurements validate the technical feasibility of a laser-driven light sail based on diffractive thin films.

Electromagnetic force and torque are typically derived from a stress tensor in conjunction with Maxwell’s equations of classical electrodynamics. In some instances, the Principle of Least Action (built around a Lagrangian) can be used to arrive at the same mathematical expressions of force and torque as those derived from a stress tensor. This paper describes some of the underlying arguments for the existence of a Lagrangian in the case of certain simple physical systems. While some formulations of electromagnetic force and torque admit a Lagrangian, there are other formulations for which a Lagrangian may not exist.

Optically levitated nanoparticles provide excellent systems to sense minute forces and explore quantum effects in a large system. However, optically levitated nanoparticles are prone to heating and require cooling and temperature stabilization to reach sensitivities necessary to study small forces and quantum effects. This problem can be solved by trapping nanocrystals doped with rare-earth ions that can be anti-Stokes cooled by tens of degrees. The efficiency of the anti-Stokes depends on gas pressure and can counter heating due to optical absorption. Cooling nanocrystals allows for thermally stabilizing nanocrystal systems to measure minute forces and quantum effects.

Recently it has been proposed to search for dark matter using mechanical sensors, exploiting the fact that all dark matter candidates couple to the size or position of atoms. While focus has been directed towards analyzing signal from gravitational wave detectors and equivalence principle tests, a unique opportunity has emerged to develop compact detectors based on cavity optomechanical systems, which have recently achieved force measurements at the quantum limit. I'll discuss this concept from an experimentalist's perspective, highlighting systems based on levitated dielectrics, silicon nitride membranes, and bulk acoustic wave resonators which are beginning to play an early role. In this context, our lab is developing a new generation of ultra-sensitive optomechanical accelerometers based on centimeter-scale silicon nitride membranes. References: [1] Manley et. al., PRL 126(6), 061301(2021) [2] Carney et. al., QST 6(2), 024002 (2021) [3] Manley et. al., PRL 124(15), 151301 (2020)

First search for new long rage forces at the micron scale using optically levitated microspheres

TBA

An optically levitated nonspherical particle in a vacuum is great for torque sensing and rotational quantum mechanics. Recently, we optically levitated silica nanodumbbells in a vacuum and drove them to rotate beyond 5 GHz. With an optically levitated nanorotor, we demonstrated a torque sensor with a record-high sensitivity [Nature Nanotechnology, 15, 89 (2020)]. Such an ultrasensitive torque sensor will have the potential to detect the long-sought quantum vacuum friction. We also demonstrated 5D cooling and observed nonlinear dynamics of an optically levitated nanodumbbell in a vacuum. This work is an important step towards full quantum control of the rigid-body motion of a levitated nonspherical particle.

Recent developments in optical trapping techniques in high vacuum allow micron-sized objects to be used as precise acceleration and force sensors with respective sensitivities of 100ng/sqrt(Hz) and 1aN/sqrt(Hz), making these sensors ideal to measure tiny forces and recoils which are the result of interactions with the environment. Due to single-electron control of the electric charge, these sensors can be made electrically neutral. This not only enables better isolation from electromagnetic noise, but also allows the measurement of electric charges on the order of 10^-4e. This levitated object is used in the search of new physics in regions of parameter space inaccessible by other techniques. This talk will discuss such sensor characterization and capabilities as well as recent results of searches for dark matter, millicharged particles bound to matter and projections for a search for short ranged interactions.

Optical vortices and circularly polarized light fields carry an orbital angular momentum (OAM), associated with their helical wavefronts, and a spin angular momentum (SAM), owing to their helical electric field. We have found that the optical vortices twist the irradiated materials to form nano/micron scale chiral or helical structures with the aid of SAM, manifesting the trajectory of their helical wavefronts. Going beyond the conventional optical trapping and manipulation, such interactions between optical vortices and materials should open the door towards new fundamental sciences and advanced technologies. Here we report on an entirely exotic interaction between optical vortices and materials, in which the optical vortex allows the ejection of a pico-liter scale spinning droplet from a donor film via laser induced forward transfer processes. This phenomenon will potentially offer us new physical insights and the next generation printing technologies to develop advanced photonic/plas

In this paper, we aim at unveiling the underlying physical mechanism for transversal optical forces, appearing due to the simultaneous illumination of a spherical object with two plane waves possessing different polarizations. The appearance of such a transversal force is quite counterintuitive since it seems to contradict the law of momentum conservation. We consider the cases of perfect electric conductor (PEC) and silver spheres illuminated by two orthogonally polarized plane waves propagating obliquely with respect to each other. Interestingly, the Poynting vector in these cases acquires a nonzero component transverse to the plane of propagation. Since the momentum transfer is related to the energy transfer, or equivalently, to non-negligible Poynting vector pointed in a particular direction, an arbitrary object placed in such external field is expected to experience a transversal force. To cast light upon this peculiar effect, we use a surface integral equation method and, along with the Maxwell stress tensor formalism, find the optical force acting on various spheres. We observe this effect for PEC spheres of different sizes and find that they are indeed subject to such transversal force. We find an explanation for this phenomenon via interference effects between selected multipoles excited in the structure. With recently developed methods, we expand the optical force into contributing pairs of selected multipoles and show that, depending on the phase between each multipole pair, the sign and direction of the force can be controlled. We also compare the results for silver and PEC spheres and find that the transversal force magnitude in silver has higher values for more limited range of sphere radii, as compared to PEC.

We report on the observation of transfer of transverse angular momentum to birefringent vaterite particles (positive uniaxial crystal) several wavelengths in size and trapped with optical tweezers in a transmission optical system and discuss prospects for quantitative measurement of angular momentum that this system poses. Such rotational control of spherical particles are excellent components of optically driven nano and micromachines that generate localised shear flows.

3D Pitch rotational motion has been generated in spherical particles using holographic optical tweezers by maneuvering the laser spots to control the rotational motion. However, since the spherical particles, required to minimise complications due to the drag forces, are perfectly isotropic, a controllable torque cannot be applied with it. It is here that we trap birefringent particles in two tweezers beams and then change the depth of one of the beam foci controllably to generate a proper pitch rotational torque wrench. We also detect the rotation with our newly developed pitch rotational motion detection technique which could not be done conventionally on isotropic spherical particles.

Spin-to-orbit conversion of light is a dynamical optical phenomenon in a non-paraxial fields, which plays an important role in various manifestations of the optical Hall Effect. Here, we demonstrate – both theoretically and experimentally – the rotational Hall Effect for a higher order Gaussian beam (HG10 ) in an optical tweezers configuration. Our theoretical results clearly reveal that for an input spin polarized HG10 mode (right/left circularly polarized), the orthogonal circularly polarized component (left/ right), generated due to angular momentum conservation following spin-orbit interaction, displays a large rotation of the intensity profile – a clear signature of the rotational Hall effect. We demonstrate the same experimentally, although the impossibility of separating out the longitudinal component from the detected intensity profile prevents us from obtaining rotation values as large as the theoretical predictions. We also measure the rotational shift as a function of the refractive index contrast in the beam path of the optical tweezers, and observe a proportional increase in general. We envisage interesting applications in inducing complex dynamics in optically trapped birefringent particles due to the spin-orbit conversion in our system.

Optical dual-beam trapping is a vital tool in biology. However, a precise rotation of large cells in 3D about arbitrary axes has not been reported yet with dual-beam traps. We introduce a novel multi-core fiber-based dual-beam optical trap using wavefront shaping. Real-time controlled rotation of HL60 cells for cancer studies is demonstrated. For the first time, the orientation of the cell can be precisely controlled about all 3D axes in a dual beam trap. Multi-core fibers provide much higher flexibility beyond the bulky optics, enabling lab-on-a-chip applications and can be easily integrated for applications like non-invasive refractive index tomography and cell-elasticity measurements.

Optical trapping is the science of holding and immobilizing particles and cells, for further manipulation and spectroscopic studies. Enhancing the application of optical trapping is limited by size and flexibility of this tool, mostly limited to high numerical aperture objectives. In this work, we show the potential of using structured light to further enhance the capabilities of optical fibres as optical tweezers, to be used for applications in which space and throughput are of importance. Using femtosecond two photon direct laser writing, we produce accurately designed micro-optic probes at the tip of optical fibres to enhance the light field for trapping single particles as well as single live cells. Enhanced trapping efficiency, ease of transporting trapped particle, and potential of performing wide spectrum spectroscopy on the trapped entity are the advantages of the present approach.

Optical traps using different configurations of optical fibers facilitate the trapping, manipulation, and characterization of particles ranging from dielectric beads, through anisotropic particles such as rare-earth doped nanorods. Here, we will introduce several such configurations, including the quasi-Bessel beam optical fiber tweezers and the optical nanofiber, and illustrate the variety of measurements that can be made depending on the configuration chosen. These optical traps provide us with unique opportunities to explore topics such as spin-orbit coupling and emission spectroscopy.

Tapered glass capillaries can be fabricated having cross-sections only few square-micrometers large. Efficient light focusing and guiding in thin, millimeter-long capillaries filled with a liquid solution provides competitive advantages for the optical sorting of nanoparticles. In this work, optical transport of fluorescent nanodiamonds is demonstrated both in water and DMSO. Size-dependent sorting of a large amount of nanodiamonds is also achieved using a backward liquid flow. An analytical model is used to calculate the femtonewton-scale forces acting on the particles and evaluate the sorting efficiency of tapered capillaries.

Three-dimensional (3D) structures with nanoscale building blocks often benefit from light-matter interaction at the nanometer size scale. The most popular existing technologies for the 3D fabrication of these small structures (e.g. direct laser writing) typically encounter significant restrictions. Here we demonstrate our optical positioning and linking (OPAL) platform for the assembly of multi-material 3D devices consisting of nanoscale components. OPAL integrates an optical trapping setup with a computer-controlled motion platform, microfluidic building block delivery apparatus, and imaging feedback system, overcoming the limitations of many traditional nanofabrication approaches, and establishing a versatile tool for the fabrication of complex 3D nanophotonic structures.

As represented by "Lab on a Tip" using microchannels, the miniaturization of manufacturing and inspection processes attracts widespread interest. Therefore, many micro-tool fabrication techniques using optical tweezers have been reported. Most reported methods focus on assembling trapped microbeads, and it was necessary to have a photocurable resin phase and surface modification process to assemble solid beads. However, because many procedures such as dispersion of microbeads and removal from the resin solution were required, flexible one-step fabrication is difficult with previous methods. This study proposed a direct adhesion and assembly of the photocurable resin droplets dispersed in the aqueous solution. Since the photocurable droplets work as both base material and an adhesive, a flexible one-step fabrication of micro-tools can be achieved. It was experimentally found that the morphology of emulsion droplets in contact significantly affects the adhesion. Generally, oil-in-water emulsion droplets are stabilized by a surfactant, and adhesion between droplets can be disturbed by a surfactant bilayer. By controlling the contact angle between the droplets, the optically trapped droplets successfully adhered together with photopolymerization. Furthermore, combining the various diameter and materials of emulsion droplets using microfluidic channels, more functional and complex microtools can be expected.

Metamaterial tweezers for trapping of polystyrene particles with 20nm diameter are presented. Proper fabrication of nanostructures on a gold thin film lead to the metamaterials that exhibit Fano interference and are being used for trapping of nanoparticles. The use of metamaterial tweezers significantly enhanced the trapping conditions and demonstrate a trapping stiffness much higher than previously reported in various cases of plasmonic tweezers.

In this paper, the use of a 2D meta-surface for the optical trapping and directed movement of spherical nanoparticles is explored. Using the Discrete Dipole Approximation, we calculate the optical forces that occur when a spherical nanoparticle moves across a surface constructed of tessellated equilateral triangles. We calculate that given sufficient optical power, choice of optical materials, and wavelengths, it is possible to use a meta-surface to guide micron sized particles in a direction dictated by the geometry of the surface. Furthermore, we suggest several experimental methods of verifying the results which are also discussed in detail.

We introduce a new class of on-chip optical tweezers with high trapping efficiency, compact footprint, and broadband operation by integrating free-form micro-reflectors and micro-lenses to the facets of waveguides to generate the strong three-dimensional optical field gradient for optical trapping. We demonstrate the design, fabrication, and measurement of both reflective and refractive micro-optical tweezers. The reflective tweezers feature a remarkably small trapping threshold power, and the refractive tweezers are handy for multi-particle trapping and inter-particle interaction analysis. This new class of tweezers is promising for on-chip sensing, cell assembly, particle dynamics analysis, and ion trapping.

Trapping and manipulation of micro and nanoparticles are essential functional elements of lab-on-a-chip and micro total analysis systems with applications ranging from single cell analysis, diagnostics, drug discovery, and chemical synthesis. Reliable optical trapping of subwavelength particles is difficult due to the diffraction limited gradient of the optical intensity. Near-field traps have emerged as excellent systems to overcome this bottleneck. Using a periodic arrangement of C-shaped apertures we demonstrate the on-chip manipulation of single sub-wavelength polystyrene beads. The wavelength and polarization sensitivity of these resonant nanostructures is exploited to expose the trapped bead to a time varying near-field force using wavelength and polarization multiplexing schemes. While optical trapping has the advantage of being contactless, dielectrophoresis provides higher forces due to the flexibility of applying larger voltages and field gradients. The RF/Microwave frequency regime where dielectrophoresis operates allows increased polarizability of certain samples due to contribution from orientational, molecular and atomic polarizations as well as particle’s electrical conductivity. Combining the above advantages of both the techniques we discuss photoinduced dielectrophoretic trapping and manipulation of nanoparticles. Using dielectrophoretic trapping we discuss the simultaneous manipulation of beads and droplets as a route for on chip chemical synthesis.

We applied a two laser extraordinary Raman spectroscopy technique in a double nanohole optical tweezers setup to excite and detect the vibrational modes of 20 nm polystyrene beads. We measured the quadrupolar accordion mode frequency to be 47.85 GHz and the spherical breathing mode frequency to be 68.72 GHz which are very close to their predicted values. According to our measurements there is a optimum power to excite the vibrational modes and to get sharp peaks in beat frequency spectrum, simultaneously. Improving the resolution of our present Raman spectroscopy system provides a way to more accurately detect changes in resonance frequency, characterize DNA unzipping, protein-DNA interactions, and classify mutant proteins.

We use machine learning (simulated annealing) to design plasmonic nanoapertures that function as optical nanotweezers. The nanoapertures have irregular shapes that are chosen by our algorithm. We present electromagnetic simulations that show that these produce stronger field enhancements and extraction energies than nanoapertures comprising double nanoholes with the same gap geometry. We show that performance is further improved by etching one or more rings into the gold surrounding the nanoaperture. We provide a direct comparison between our design and work that is representative of the state of the art in plasmonic nanotweezers at the time of writing. Lastly, we provide experimental results that compare our algorithm-designed structure to a double nanohole design

We introduce a toolbox to calibrate microscopic force fields by analyzing the trajectories of a Brownian particle using machine learning, namely, recurrent neural networks. We demonstrate that this machine-learning approach outperforms standard methods when characterizing the force fields generated by harmonic potentials if the available data are limited. More importantly, it provides a tool to calibrate force fields in situations for which there are no standard methods, such as non-conservative and time-varying force fields. In order to make this method readily available for other users, we provide a Python software package named DeepCalib, which can be easily personalized and optimized for specific force fields and applications.

Low-power and non-invasive optical manipulation of tiny objects is important in both material science and life science. Harnessing the high-efficiency light-to-heat conversion and the thermophoretic migration under a temperature gradient, we explore the opto-thermo-mechanic coupling during light-matter interaction and develop an indirect optical manipulation technique termed opto-thermophoresis, which features low operation power and simple optics. Based on this concept, we development new tools such as opto-thermophoretic tweezers, printers, tractors, and motors. Further, we demonstrate these optical tools in different fields, such as biomedical manipulation, bio-sensing, and reconfigurable optics.

The flourishing field of light-powered micro/nanorotors provides promising strategies for manufacturing and biomedical needs. However, the torque of optical rotors typically arises from the momentum exchange with photons, which limits the geometries and materials of objects that can be rotated and requires intense laser beams with designed intensity profile and polarization. These factors inhibit the light-powered rotation of highly symmetric or isotropic targets. Herein, we developed an optothermal micro/nanorotors platform that enables the rotation of various colloids with diverse sizes, materials, and various shapes, including live cells and micro/nanoparticles with high symmetry and isotropy. The long-sought-after out-of-plane rotation has been achieved by a single plane-polarized Gaussian laser beam with an ultralow power. This simple rotor approach is foreseen to open new horizons in colloidal and life sciences by offering a non-invasive and universal manipulation.

Integration of plasmonic nanotweezers on silicon chips holds promise for many lab-on-a-chip applications. In this work, optical trapping of single beads and self-assembled clusters composed of up to nine beads is achieved using plasmonic nanotweezers fabricated on a silicon nanophotonic waveguide. We statistically analyze the preferred orientations, the probability of occurrence and the stability of the different cluster configurations. We evidence that the presence of two potential barriers along the waveguide edges results in the enhanced stability of some specific cluster configurations. Our results pave the way for further development of hybrid plasmonic-photonic nanotweezers and lab-on-chips.

We develop a new type of optical tweezers that exploits thermophoresis and solid-state optical refrigeration to trap and manipulate nanoparticles at the laser-generated cold region. Localized optical refrigeration of a solid substrate creates a temperature gradient field to trap nanoparticles and molecules with common thermophobic nature at the laser beam center by thermophoresis. The trapping of objects at the low-temperature region can effectively avoid photothermal degradation. In addition, compared to conventional optical tweezers, our tweezers require a weakly focused laser beam with a lower optical intensity to trap fragile objects, which can reduce the photodamages.

Since Arthur Ashkin and coworkers found that focused laser beam could displace and levitate microsized particles, optical tweezers has turn out to be a reliable noncontact tool for 3D manipulation of micro-objects, allows sensing by using only a single particle. The further development of nanophotonics toward higher sensitivities and resolutions continues to stimulate optical trapping of smaller and smaller objects. While applied to sub 100 nm-particles, the optical force starts to get insufficient to trap or manipulate, and the trap potential starts to be comparable to the thermal energy. Although there are a lot of methods that have been proposed to enhance the optical force, few research gets insight of the temperature effect on optical trapping. In this work, we summarize our recent experimental results on thermal sensing experiments in which micro/nanoparticles are used as probes with the aim of providing a contemporary state-of-the-art about the temperature effects in the stability of potential trapping processes.

Spatiotemporal control refers to the simultaneous control over the position and time variables during the dynamics of an event. The monitoring of fundamental processes at microscopic lengths and ultrashort timescales is typically intertwined with the interaction of light with matter. Optical trapping for spatial control results in the immobilization of microscopic objects with light pressure from a tightly focused laser beam and has wide-ranging applications. Conventional single-beam optical tweezers use continuous-wave lasers for spatial control through photon flux but are lacking in temporal control aspects. Pulsed lasers can control the dynamics of the process being studied and thus provide a temporal control knob by enabling the circumvention of relaxation processes. The tradeoff, however, is that this spatial control knob becomes more dependent on the frequency of the light source. The high photon flux requirement of stable optical tweezers with pulsed lasers necessitates femtosecond pulse shaping at a rapid repetition rate, which has been a barrier until recently. We present a generalized model to describe thermal effects for optical tweezers, capable of treating pulsed lasers systems (which differ in instantaneous interaction effects) as well as CW systems (which can be understood as a limiting case of our model). The model has nonlinear optical (NLO) interactions included prima-facie and can describe pulsed laser tweezers in areas where they excel, like the two-photon-fluorescence-based detection. A key result obtained from our model is the fact that NLO interactions can be used to balance out thermal effects.

We study theoretically the opto-mechanics of a metallic nano-shell with a gain-enriched dielectric core in stationary Optical Tweezers. In order to avoid the counterproductive effects of scattering forces we choose a two counter-propagating beams configuration. The application of an external pump enhances the plasmonic resonance of the nano-shell thus affecting the optical forces acting on the particle even at pump powers below the emission threshold. We show that the trapping strength can be largely improved without the necessity to increase the trapping beam power. We support the theoretical analysis with Brownian dynamics simulations that show how particle position locking is achieved at high gains in exended optical trapping potentials. Finally, for wavelengths blue-detuned with respect to the plasmon-enhanced resonance, we observe particle channeling by the standing wave antinodes due to gradient force reversal.

We demonstrate a novel intracavity optical trapping approach based on a self-adjusted counter propagation beam. In this configuration, the trapping part is placed within a backward pumping fiber ring cavity. The particle in this setup is trapped by two counter propagation beams including pump, from top to bottom and signal from bottom to top. This trapping scheme benefits from the cavity design in which the individual beams are adjusted depending on the displacement of the trapped particle. When the particle is in the center of the trap and turns the laser signal off, the pump then automatically turns on and this keeps the particle in the trap position. We believe this approach can find its applications in the trapping of submicron particles where the previous intracavity OTs were not able to trap, especially biological samples that are subjected to photo-damage due to the intensity of laser light.

Here we use optical trapping to isolate single Yb/Er-doped upconversion nanocrystals in plasmonic double nanohole apertures and show that the geometry of the aperture can be tuned to give high emission rate en- hancement. The double nanohole apertures show additional enhancement over the rectangular apertures that were previously demonstrated by our group, producing enough enhancement to observe emission at 400 nm and 1550 nm with 980 nm excitation—not seen in our group’s previous work with rectangular apertures. A facile method for tuning the geometry of double nanohole apertures by adjusting the plasma etching time in the colloidal lithography fabrication process is discussed. We find that a double nanohole with a cusp separation of 32 nm yields the greatest emission enhancement with multiple plasmonic resonances which enhance both the excitation and emission wavelengths. The emission enhancement for the DNH with 32 nm cusp separation was found to be a factor of 54, 44, and 31 greater than the rectangular apertures used in our group’s previous work, for wavelengths of 650 nm, 550 nm, and 400 nm. This result shows that double nanohole apertures can be tuned for emission enhancement as required by specific applications.

Nonlinear optics provides the functionality of wavelength conversion and switching that allows for photonic signal processing and bioimaging. A major obstacle for extreme photonic integration is the requirement to use ultrafast pulsed lasers or to work above the diffraction limit to activate nonlinear optical processes. Here we show that plasmonic enhancement and two dimensional materials provide a promising pathway to extreme photonic integration with nonlinear functionality in nanoscale while using diode lasers.

The trapping and dynamic manipulation of nanoscale biological molecules such as proteins and exosomes using the conventional optical tweezer technology have been met with challenges. This arises because of the substantial optical intensity required, which poses photo-induced damage to the biomolecules. In this talk, I will introduce a new kind of optically controlled nanotweezers termed Opto-Thermo-Electrohydrodynamic Tweezers (OTET) that enables the stable trapping and dynamic manipulation of sub-10 nm biomolecules at locations that are several microns away from the high-intensity laser focus, where they experience both negligible photothermal heating and light intensity. This novel non-invasive optical nanotweezer is expected to open new horizons in life science and medicine by offering an unprecedented level of control of tiny nano-sized biological objects in solution without photo-induced damage.

The precise spatio-temporal manipulation of droplets is fundamental for many lab-on-a-chip systems with applications in biology, healthcare and chemistry. Different approaches have been investigated, including thermal, chemical and electrical methodologies. Among this latter, electrophoresis (EP) and dielectrophoresis (DEP) play a key role, since they are highly compatible with microfluidic systems and provide sufficiently strong forces to control up to microliter volume aqueous droplets. However, EP and DEP techniques typically require the presence of metallic electrodes to create the desired electric fields, making these approaches less flexible and efficient than those exploiting pure optical techniques. Iron-doped lithium niobate (LiNbO₃:Fe) allows for the generation of strong electric field modulation due to an inhomogenous illumination, thanks to its photovoltaic properties. These photoinduced fields interact as EP and DEP forces with microdroplets, while guaranteeing the flexibility provided by optical field-based modulation. Indeed, the combination with well-known techniques to control and modulate light fields can be exploited to generate virtual electrodes on the material, achieving reliable as well as flexible devices for water droplets control. In our approach, the photoinduced fields generated by the complex illumination of LiNbO₃:Fe are exploited to control motion and trajectory of water droplets inside microfluidic channel. Moreover, the crystal is integrated in standard droplet microfluidic polymeric device, substituting the usual glass substrate and, thus without hindering the portability. This feature combined with the control of positions of aqueous droplets represents a key tool for several applications of customized lab-on-a-chip systems, highlighting the capabilities of LinbO₃:Fe-based virtual electrodes.

We present a powerful and efficient technique to structure acoustic fields, based on the use of active diffraction elements, which can be designed in practically arbitrary shapes and can be operated within a broad spectral range of ultrasonic frequencies in air. Two kinds of structured fields are demonstrated: focused acoustic vortices and the simultaneous generation of multiple acoustic Bessel beams of different topological charges, well separated among each other along the propagation axis. In both cases, the main parameters of the field can be finely and continuously tuned by setting the operation frequency. Experimental results show very good agreement with theoretical analysis and numerical simulations.

Photophoresis can trap opaque microscopic particles in a focused laser beam surrounded by a gas such as air. The particle is heated by the laser, and in turn, interactions with the ambient gas provide a stabilizing force that holds the particle in a specific region of the beam. The particles can stay trapped while the beam ismoved side to side up to 2 m/s, enabling three-dimensional images to be traced out in a display application. Structure in the laser beam is associated with the trapping phenomenon, but the fundamental mechanism for stability of the trap remains mysterious. Particles prefer regions of the beam with diffraction features such as those that arise from spherical aberration. Nevertheless, the ability of near-unidirectional light, albeit light that undergoes focusing and exhibits structure, to provide a restoring force to trapped particles in the direction opposite to beam propagation needs to be explained. Through repeated trials of capturing particles in a well characterized beam, we map out the preferred locations for particle capture and correlate them with diffraction features of the beam. The specific beam locations that host trapped particles, when compared with neighboring regions that do not, can offer insight into the stability mechanism. We analyze the Poynting vector in the vicinity of trapped particles. The flow of light energy can provide important clues into the trapping mechanism.

We analyze the trapping potential of a setup of counter propagating optical tweezers and test the validity of the harmonic approximation in the dimension parallel to the trapping beam axis. While the dynamics of spherical droplets is very well described by a harmonic oscillator, we find that the trapping potential changes drastically for particles at resonance with the trapping laser, at so called Mie resonances. Our findings are discussed with regard to possible origins of this phenomenon.

The evaporation of liquid droplets was measured using a counter-propagating optical trap where trapping is stable from 6 um down to complete evaporation. The droplets' Mie scattering was used to calculate their size as they evaporated exponentially during 5-10 s. Simultaneously, the scattering intensity was recorded and a full Whispering Gallery Mode spectrum with around 100 resonances was observed, providing an alternative higher resolution measurement of the evaporation process. Droplets of different solutions were readily evaporated at room temperature and pressure. Importantly, this technique imposes no special requirement on the environment. It could be expanded to study evaporations of other liquids or in different humidities, temperatures, and pressures.

Marine plastic debris is a widely recognized environmental issue. By employing an optical micro-Raman tweezers setup, we have identified the composition of particles trapped in marine aggregates collected from the coastal surface waters around the subtropical island of Okinawa, Japan. This chemical identification of small microplas- tics at the single-particle level contributes to extending our knowledge of plastic pollution in the ocean around a Blue Zone region.

Manipulation of biological cells using optical trapping is a non-invasive approach in which individual living cells are examined without causing any damage because there is no direct mechanical contact with cells. Optical trapping uses a tightly focused laser beam emitted through a high numerical aperture microscope objective lens to hold microscopic particles. When using this technique, there is minimal chances of exposing cells to contamination and optically handled cells can still be utilised in downstream sterile experiments whenever necessary. In this study, optical trapping is used to trap HIV infected cells, which are then analysed by Raman spectroscopy. Raman spectroscopy as an analytical technique provides specific chemical/molecular details about a sample based on the fundamental vibrational modes of the chemicals. By combining these two light-based technologies, HIV infected TZM-bl cells were distinguished from the uninfected cells as they exhibited different molecular fingerprints. The acquired results both confirm and provide more detail to the findings of the previous study where transmission spectroscopy was used to differentiate between HIV infected and uninfected cells. This current study shows how the two cell populations differ according to the chemical/molecular composition and distribution. These results present valuable information that would be essential in the development of a label-free HIV point of care diagnostic device.

Cosmic dust particles are usually collected in space or in the Earth’s stratosphere and deposited on a substrate to be analysed at large terrestrial facilities. We use Raman tweezers technique for the contacless manypulation of cosmic dust particles, to identify their compositions and to characterize their response to optical forces without any substrate effects, documenting the high potential of this novel technique for space exploration.

Self-organisation is driven by the interactions between the individual components of a system mediated by the environment, and is one of the most important strategies used by many biological systems to develop complex and functional structures. Furthermore, biologically-inspired self-organisation offers opportunities to develop the next generation of materials and devices for electronics, photonics and nanotechnology. In this work, we demonstrate experimentally that a system of Janus particles (silica microspheres half-coated with gold) aggregates into clusters in the presence of a Gaussian optical potential and disaggregates when the optical potential is switched off. We show that the underlying mechanism is the existence of a hydrodynamic flow induced by a temperature gradient generated by the light absorption at the metallic patches on the Janus particles. We also perform simulations, which agree well with the experiments and whose results permit us to clarify the underlying mechanism. The possibility of hydrodynamic-flux-induced reversible clustering may have applications in the fields of drug delivery, cargo transport, bioremediation and biopatterning.

Optical trapping of janus particles has turned out to be complicated due to lack of control on the direction of orientation. Here we use an alternative strategy, where we optically trap a NaYF₄:Yb,Er upconverting nanoparticle on the pump wavelength at 975 nm and show that there is much greater visible emission in the backscatter direction than the forward scattered direction, leading to greater heating in the backscatter region of the nanoparticle. This then generates a temperature gradient across the nanoparticle to push it in the axial direction. The Mean Square Displacement (MSD) bears signature of the Hot Brownian Motion (HBM) when trapped at 975 nm, which becomes regular diffusive when the trapping wavelength is changed to non-pump wavelength 1064 nm. The effective velocity of the particle while trapped in the tweezers can be directly estimated. Thus, this is the first time that an active janus-like particle has been optically trapped in tweezers.

Optical tweezers are a powerful tool in biophysics due to the ability to manipulate single biomolecules [1,2]. For dual-trap tweezers with a steerable beam and force detection via back focal plane interferometry and a position sensitive detector [3,4], calibration can be done by stretching single DNA molecules [5,6]. Typically, DNA stretching provides accurate values for 𝛼, the force scale factor that converts from bead displacement from the trap to force, and 𝛽, the conversion between rotation of the piezoelectric controlled mirror and the linear displacement of the movable trap. Determining trap compliance using this method relies on nonlinear fitting that may be sensitive to noise and drift

Laser induced shockwave (LIS) can be utilized to subject neuronal cells to conditions similar to those occurring during a blast induced traumatic brain injury. We utilized a 532nm Coherent Flare laser to induce a shockwave near cells which had been transfected with a FRET calcium biosensor (D3CPV) so that we could monitor the immediate cellular responses. Our shockwave system was characterized with a high-speed camera to monitor cavitation bubble dynamics and calculate the shear forces cells were subjected to. We found that we could induce forces which have been previously shown to induce injury. Using both phase and fluorescence microscopy we monitored the effects of shear on our cells. We found that at distances up to 120 microns from the laser focal point cells experienced shears greater than 10kPa. At those distances cell fragmentation was observed. Cells that survived and expressed the FRET biosensor demonstrated an immediate calcium elevation irrespective of extracellular or cytoplasmic calcium concentration. Cells recovered to pre-shockwave calcium levels within ~30s. In conclusion, LIS can be utilized to simultaneously monitor the neuronal response to shear stress and nearby cell death or injury.

We report an experimental demonstration of spectral compression performed on heralded 795 nm single photons with narrow spectral bandwidths about 3 times larger than the corresponding atomic transitions, generated through four-wave mixing in cold Rubidium-87 atoms. This scheme uses an asymmetric cavity as a dispersion medium to spread out the photon temporally, and subsequently a phase modulator, to modify the spectral energy distribution of the photons. The spectral bandwidth of the photons was compressed by a factor of 2.6, from 20.6 MHz to less than 8 MHz, almost matching the corresponding atomic transition linewidth of 6 MHz. The spectral compression scheme can potentially be adapted to a wide range of wavelengths and spectral widths.

Axial optical chain (optical bottle beams) beams are widely used in optical micromanipulation, atom trapping, guiding and binding of microparticles and biological cells, etc. However, the generation of axial optical chain beams are not very flexible at present, and its important characteristics such as periodicity cannot be easily regulated. Here, we propose a novel method to achieve the axial optical chain beams with controllable periodicity and phase. Axial optical chain beams is produced by interference of two zero-order Bessel beams with different axial wave vectors. The periodicity of the axial optical chain beams can be changed by changing the radius of the double annular slits, respectively. The feasibility and effectiveness of the proposed method are demonstrated by theoretical numerical analysis. This method will further expand the application of axial optical chain beams in optical tweezers, optical modulation and other fields.

Under short pulsed excitation, depending on the nonlinear refractive index of the particle and the surrounding medium, optical nonlinearity plays a significant role in modulating optical forces on particles. Here, we explore the trapping forces experienced by dielectric nanoparticles in a highly nonlinear medium taking into account nonlinear propagation of Gaussian beam.

We present 3D steady-state trapping and manipulation of vapor microbubbles in pure ethanol by optothermal effects induced by low power CW laser. Light absorption from silver nanoparticles photodeposited on the core of a multi-mode optical fiber is used as a heat source to induce microbubbles of several diameters. The optothermal effects generated by the light absorption of light bulk absorption modulates the microbubble wall's surface tension inducing both longitudinal and transversal forces just like optical forces, generating a 3D potential well. By numerical simulations, we find expressions to describe both the temperature profiles and the convective currents. Besides, using an array of three optical fibers with silver nanoparticles photodeposited on their cores we show the transmission of microbubbles from one fiber to another by switching the laser radiation from the CW lasers.

We present the massive migration of metallic nanoparticles (NPs) due to their interaction with thermal gradients. A single-mode optical fiber is introduced into an ethanol solution with silver NPs of size between 0.5 1 µm- radius. When the laser radiation of λ = 455 nm leaving the optical fiber illuminates the suspended silver NPs, they absorb part of the energy and remit it as heat to the surrounding environment. This causes the formation of a thermal gradient in each particle and the medium. When the NPs are near the temperature gradient generated in the medium, they move to the high or low-temperature zone, the direction depending on the distance between the NPs and the tip of the optical fiber. The phenomenon that describes this behavior is called ∆α photophoresis. The particles present a considerable absorption on the illuminated side, generating a temperature difference relative to the non-illuminated side. The heat is conducted through the medium that surrounds the particles, giving rise to a momentum exchange between the molecules of the medium and the NPs. This exchange causes either an attraction or a repulsion of the NPs relative to the end optical fiber. The displacement of the NPs stems from the influence of the ∆T force, whose direction is equal to the propagation of the beam, and the force ∆α, with the opposite direction on each NP. We also study the appropriate optical and thermal conditions that enhance the migration of the silver NPs.

In this work, we study how to grow biofilms and control their growth with optical trapping techniques. In low nutrition media Bacillus subtilis (NCIB 3610) secretes extracellular polymeric substance (EPS), glue-like substance, and ultimately form a biofilm. To observe cells aggregation into clusters and the dynamics of controlled biofilm formation, we utilize an optical tweezers system built around a commercial inverted microscope. A highly focused near-infrared laser beam generated by our optical tweezers attracts cells toward the center of the laser beam, where they are trapped into clusters.